Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over adsorbent beds which preferentially adsorb a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated to a higher pressure during intervals of flow in a first direction through the adsorbent bed, and is reduced to a lower pressure during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed or "light" component is concentrated in the first direction, while the more readily adsorbed or "heavy" component is concentrated in the reverse direction.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valves at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. Valves are required to control feed gas admission and discharge of gas enriched in the heavy component at the feed ends of the adsorbent beds, to control delivery of gas enriched in the light component at the product ends of the adsorbent beds, and to control depressurization and repressurization steps from either the feed or product ends of the beds.
Enhanced separation performance is achieved in well known PSA cycles using steps for each adsorbent bed of cocurrent feed at the higher cycle pressure, cocurrent initial blowdown, countercurrent final blowdown, countercurrent purge at the lower cycle pressure, and countercurrent pressurization. As disclosed by Kiyonaga (U.S. Pat. No. 3,176,444), Wagner (U.S. Pat. No. 3,430,418) and Fuderer et al (U.S. Pat. No. 3,986,849), improved product recovery can be obtained with more than two adsorbent beds operating in parallel, by performing pressure equalization steps between the separate beds so that a first bed undergoing a pressure reduction step exchanges gas which typically has been substantially purified to a second bed undergoing a pressure increase step so that the working pressure of the first and second beds is equalized to a pressure intermediate between the high and low pressures of the cycle.
With a greater number of beds, multiple pressure equalization steps can be achieved, although the valve logic and controls are then greatly complicated. Modern industrial scale PSA plants with six or more beds (e.g. as described by Fuderer et al for hydrogen purification) use a large number of two-way valves under computer control to establish both the cycle switching logic and adaptive flow control of each step.
It is well known that the complexity of valving in PSA systems may be reduced by use of multiport valves to establish the cycle switching logic. Thus, Snyder (U.S. Pat. No. 4,272,265) has disclosed a rotary distributor valve for controlling high pressure feed and low pressure exhaust flows for an air separation pressure swing adsorption system with multiple beds. Use of a coaxially aligned pair of distributor valves, respectively controlling feed and product gas flows at opposite ends of the beds, was disclosed by van Weenen (U.S. Pat. No. 4,469,494). Hill (U.S. Pat. No. 5,112,367) and Hill et al (U.S. Pat. Nos. 5,268,021 and 5,366,541) have disclosed oxygen concentration PSA devices using multiport rotary valves with stationary adsorbent beds. The processes disclosed by van Weenan and Hill have pressure equalization steps conducted at respectively the product or feed ends of the adsorbent beds.
Prior art PSA systems with multiport distributor valves have been used commercially in small scale oxygen enrichment applications, as recommended by Dangieri et al (U.S. Pat. No. 4,406,675) for a rapid PSA process in which flow control is intentionally established by relatively steep pressure gradients in the adsorbent bed. The adsorbent bed must therefore be spring-loaded or otherwise immobilized to prevent attritional damage.
For large industrial PSA systems, mechanical immobilization of the adsorbent beds has not been practicable. Careful flow control is required to ensure that pressure gradients in the adsorbent bed are kept low, well below the onset of fluidization.
Mattia (U.S. Pat. No. 4,452,612) and Boudet et al (U.S. Pat. No. 5,133,784) disclose PSA devices using a rotary adsorbent bed configuration. The multiple adsorbent bed ports of an adsorbent bed rotor sweep past fixed ports for feed admission, product delivery and pressure equalization; with the relative rotation of the ports providing the function of a rotary distributor valve. Related devices are disclosed by Kagimoto et al (U.S. Pat. No. 5,248,325). All of these prior art devices use multiple adsorbent beds in parallel and operating sequentially on the same cycle, with multiport distributor rotary valves for controlling gas flows to, from and between the adsorbent beds.
An advantage of PSA devices with the adsorbent beds mounted on a rotary adsorbent bed assembly, as in the cited prior art inventions by Mattia and Boudet et al., is that function port connections for feed, exhaust, product and pressure equalization are made to the stator and are thus accessible to flow control devices. However, a rotary adsorbent bed assembly may be impracticable for large PSA units, owing to the weight of the rotating assembly. Also, when separating gas components which are highly inflammable or toxic, the rotary adsorbent bed assembly would need to be completely enclosed in a containment shroud to capture any leakage from large diameter rotary seals. Hence, PSA devices with stationary adsorbent beds will be preferred for larger scale systems, and for applications processing hazardous gases such as hydrogen.
In some of the above referenced prior art (e.g. Mattia, Boudet, and van Weenan), the rotary distributor valve would rotate continuously. Lywood (U.S. Pat. No. 4,758,253) and Kai et al (U.S. Pat. No. 5,256,174) have mentioned intermittent actuation of rotary multiport distributor valves for PSA systems, so that the distributor valve is stopped at a fully open position during each step of the cycle, and the distributor valve is then switched quickly to its next fully open position for the next step of the cycle.
It will be apparent that the multiport valves disclosed in the above cited inventions enable a simplification of PSA cycle switching logic, particularly those using multiple beds with pressure equalization steps, since the control functions of a multiplicity of two-way valves are consolidated into one or two multiport distributor valves. However, these prior art devices have limited utility except in small scale applications, owing to their lack of control flexibility. Since valve timing logic and port orifice sizing of the multiport valves are fixed rigidly in these prior art inventions, there is no provision for flow control to provide operational adjustment under changing feed conditions or during intervals of reduced product demand, or for performance optimization.
This inflexibility of control is most limiting for those of the cited prior art inventions which use multiport valves to exchange gas between a pair of beds, and across a pressure difference between that pair of beds. Such gas exchanges between pairs of beds arise in pressure equalization steps, in purge steps, and in product repressurization steps. For the PSA cycle to operate properly in a given application between given high and low pressures of the cycle, a correct amount of gas must be exchanged between a pair of beds in each such step, across the continuously changing pressure difference between that pair of beds during the step, and over the time interval of that step.
Especially in large industrial PSA systems, it is also necessary to avoid high velocity transients that could damage the adsorbent by excessive pressure gradients or fluidization. Such transients could occur as valve ports open at the beginning of an equalization or blowdown step. The internal geometry and orifice dimensions of a multiport distributor valve govern the amount of gas which can flow across a given pressure gradient over a given time interval. Once the internal orifice apertures of the rotary valves and piping connections have been fixed, the prior art PSA cycle using multiport valves could only operate correctly between given high and low pressures at one cycle frequency with a given feed composition, and would have no means for operational adjustment to optimize cycle performance.
Hence, prior art PSA devices with multiport valves would be unable to operate at much reduced cycle frequency during periods of reduced demand for purified product. It would be highly desirable to reduce cycle frequency when product demand is reduced, since lower frequency operation would be more efficient at lower flows, less stressful on the adsorbent and valve components, and less noisy in medical applications.
The ability to adjust operating frequency is also vital for applications where a product purity specification must be satisfied, while the highest attainable product recovery is desired from a feed mixture of given composition and flow rate and working between given higher and lower pressures. If the cycle frequency is too slow, the apparatus will release a relatively small exhaust flow at the lower pressure, resulting in high recovery of the light product at less than specified purity. If the cycle frequency is moderately too high, the apparatus will release a larger exhaust flow, achieving higher than desired purity and lower than desired recovery of the light product. If the cycle frequency is much too high, mass transfer effects may degrade performance to result in unsatisfactory light product purity as well as low recovery. Such applications arise for example in industrial hydrogen purification. In these applications, cycle frequency must be adjustable in order to achieve specified purity and simultaneously high recovery of the light product.
None of the cited prior art for pressure swing adsorption with multiport valves addresses the combined need for adjustable cycle frequency control and adjustable flow controls for gas exchanges between pairs of adsorbent beds. There is no flow control other than the pressure drop resistance of the conduits and the valve ports as they open and close. Hence, these devices as disclosed have the operational limitation that they cannot be operated at significantly varied conditions of cycle frequency and pressure.
It is well known that there is much scope for optimization of PSA cycles by adjusting the pressure intervals taken up by different steps. For example, Suh and Wankat (AIChE Journal 35, pages 523-526, 1989) have published computer simulation results showing the sensitivity to adjustment between the pressure intervals allocated to cocurrent and countercurrent blowdown. They showed that the optimum split between the pressure intervals for cocurrent and countercurrent blowdown is sensitive to the feed gas composition and the adsorbent selectivity. Product recovery performance is degraded by operation away from the optimum operating point.
The above cited PSA devices with multiport distributor valves lack any control means for making adjustments between the pressure intervals taken up by the different steps of the cycle. It would be very desirable to provide a control system capable of such adjustment while the PSA system is operating.
A further limitation of the prior art for PSA devices using multiport valves is the lack of control means to establish relatively smooth and constant flow over each step. Such control means could usefully alleviate the flow inrush at the beginning of each step when valve ports open across pressure differences, thus protecting the adsorbent bed and valve ports from transient flow velocities much in excess of the average flow during each step. Such control means could also minimize the time intervals of zero or much below average flow velocity during valve switching between steps, thus enhancing the productivity of the apparatus.